In circuit arrangements which have a power transistor for switching or regulating a current flowing through a load, it may be necessary to measure a load current flowing through the power transistor. A sense transistor is provided which is connected in such a way that it is operated at approximately the same source, gate, and drain voltages as the power transistor, i.e., the same operating point. The current through the sense transistor is then directly related to the load current; the ratio of the currents through the power and the sense transistors is called KILIS (“K”):
  K  =                    I        load                    I        sense              .  
If operated at exactly the same operating point, K will remain constant. However, due to manufacturing differences, the transistors are not operated at the same operating point. K will have a “spread” between a minimum and a maximum value. As the accuracy of K is important for the circuit arrangement, this spread should be kept within prescribed bounds. FIG. 7 illustrates typical diagnosis behavior of K for a diagnosis current proportional to a load current under different operating conditions. In particular, maximum and minimum values of K at various load currents are shown, indicating the spread of K.
With gate back regulation (“GBR”) the power and sense transistors are operated at a constant drain voltage. It is common to use GBR to improve accuracy at low load currents. The principle is to regulate the drain-source voltage to a constant voltage by decreasing the gate voltage.
In systems with GBR, sudden activation of a high-current load can cause a load step (i.e., a current jump) of sufficient magnitude resulting in a significant voltage drop over a corresponding module switch. The normal lag in operation of GBR during a load step with such a voltage drop may inadvertently trigger an under-voltage shutdown in a microcontroller. This case is illustrated in FIGS. 8A and 8B, in which a circuit arrangement 800 includes a module 801, a pre-regulator 804, a microcontroller 803, and a load switch 802. In case module 801 is turned on, the current (IOUT in FIGS. 8A and 8B) may be low, such that module 801 operates in GBR mode. Load switch 802 is turned on, causing VOUT to decrease, which requires some time to return to its previous level. An undervoltage condition, as seen in FIG. 8B, could then occur.
Additionally, power consumption may be affected by whether GBR mode is used. This is illustrated in FIG. 9, which compares power consumption for various currents with and without GBR mode.